1. Field of the Invention
The present invention relates to an image forming apparatus and a control method therefor, and more particularly, to an image forming apparatus having a plurality of image forming units and a control method therefor.
2. Description of the Related Art
Conventionally, a color image forming apparatus of this type has been proposed, which has a plurality of image forming units which each operate such that a laser beam which has been light-modulated according to recorded information is irradiated on a photosensitive drum, an electrostatic latent image on the photosensitive drum is developed and transferred onto a transfer sheet or an intermediate transfer belt using an electrophotographic method. In the color image forming apparatus, a transfer sheet is sequentially conveyed to the image forming units by means of a transfer material conveying belt, so that latent images on the photosensitive drums are overlappingly transferred onto the transfer sheet, or alternatively the latent images on the photosensitive drums are overlappingly transferred onto the intermediate transfer belt and then the transferred images are collectively transferred onto the transfer sheet, to thereby form a color image.
The color image forming apparatus of this type, however, has the disadvantage that the final overlapping images transferred onto the transfer sheet do not align with each other (mis-registration), due to mechanical installation errors of the photosensitive drums, variations in optical path length between the laser beams, and changes in the optical paths.
To overcome this disadvantage, there has been known a method in which a pattern image for registration correction which is transferred from the photosensitive drums onto the transfer belt is read by CCD or PD sensors provided at the respective photosensitive drums, and time differences in reading the image pattern between the CCD or PD sensors are detected in advance as mis-registration values for the respective color images, based upon which the image forming units for the respective color images are controlled so as to be synchronized in transfer timing with each other, to thereby perform image formation (see Japanese Laid-Open Patent Publication (Kokai) No. H6-051607, for example). When this method is used, however, the scanners of the image forming units have to be first started for synchronization before the start of a preparation for image formation of the image forming apparatus.
Further, the image forming apparatus of this type can be used to form an image using only one particular image forming unit. For example, a black and white image alone can be outputted.
When a single color image such as a black and white image is thus outputted, however, if all the image forming units are operated to make preparations for image formation, the output of image formation may get delayed, and the image forming units, which are not used for forming the image, are also activated into an image formation enabling state, resulting in that the image forming units become worn.
Conversely, if only the image forming unit that forms black and white images is activated into an image formation enabling state, if it is necessary to output a color image during execution of the image formation, the image forming units required for forming the color image are activated into the image formation enabling state, which results in a prolonged period of time before the color image is outputted. This will be explained in further detail with reference to FIG. 8.
FIG. 8 is a timing chart showing a control process for switching from an image forming operation for a black and white image to an image forming operation for a color image carried out by the conventional image forming apparatus.
When a signal for starting the image forming operation is issued, the photosensitive drums of all of the image forming units and the intermediate transfer unit are caused to start to be driven, and a scanner 13a of the image forming unit a for the black and white image also starts to be driven. The scanner 13a is accelerated to a predetermined rotational speed over a time period Ts1 and is then controlled to rotate at a constant rotational speed. After the scanner 13a starts to be driven, a preparation for image formation by the image forming unit a for the black and white image is started.
In the present example, it is assumed that the image forming unit a for the black and white image is disposed at such a position that it is the last image forming unit to carry out the transfer of the image onto the intermediate transfer member. Once the image forming unit a becomes ready for the image formation, image forming operations I1, I2 are started.
After the image forming operations I1, I2 for black and white images are completed, image forming operations I3d, I3c, I3b, I3a for color images are carried out. That is, after completion of the image formation I2, scanners 13b, 13c, 13d are activated to start preparations for image formation by the remaining image forming units b, c, d. The scanners 13b, 13c, 13d are then accelerated to a predetermined rotational speed over a time period Ts1 and are then controlled to rotate a constant rotational speed. Once the constant rotational speed has been reached, a synchronization operation (Ts2) is performed such that the scanners 13a to 13d are rotatively driven with a certain angular phase difference between them.
The preparation for image formation by the image forming unit b is started in such timing that the preparation is completed before the synchronization of the scanners 13a to 13d is completed (a time period Thd is given for the preparation for image formation). After the preparation for image formation by the image forming unit d is started, a preparation for image formation by the image forming unit c is started after a time interval Tst. Then, a preparation for image formation by the image forming unit b is started after a time interval Tst. The image forming operation I3d by the image forming unit d is started, once the preparation for image formation by the image forming unit d and the synchronization operation for the scanners 13a to 13d are completed. After the image forming operation I3d is started, image forming operations I3c, I3b, I3a by the image forming units c, b, a sequentially take place with a time interval Tst between the image forming operations. Thus, the image forming units b, c, d are required to be brought into the state for the image formation in color, which leads to a delay in the output of the image.